Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely proposed for large-scale energy storage applications. Various battery designs, including flow batteries, have been considered for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing sides of a membrane or separator in an electrochemical cell containing negative and positive electrodes. The flow battery is charged or discharged through electrochemical reactions of the active materials that occur inside the two half-cells. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or variants thereof will synonymously refer to materials that undergo a change in oxidation state during operation of a flow battery or like electrochemical energy storage system (i.e., during charging or discharging). Although flow batteries hold significant promise for large-scale energy storage applications, they have often been plagued by poorer than expected energy storage performance (e.g., round trip energy efficiency) and limited cycle life, among other factors. Despite significant investigational efforts, no commercially viable flow battery technologies have yet been developed.
Metal-based active materials can often be desirable for use in flow batteries and other electrochemical energy storage systems. Although non-ligated metal ions (e.g., dissolved salts of a redox-active metal) can be used as an active material, it can often be more desirable to utilize coordination compounds for this purpose. As used herein, the terms “coordination complex,” “coordination compound,” “metal-ligand complex” and related variants thereof will synonymously refer to a compound having at least one dative bond formed between a metal center and a donor ligand. The metal center can cycle between an oxidized form and a reduced form in an electrolyte solution, where the oxidized and reduced forms represent states of full charge or full discharge depending upon the particular half-cell in which the coordination compound is present.
Due to their favorable electrochemical properties, titanium coordination complexes can be particularly useful for inclusion as at least one of the active materials within in a flow battery, particularly as the active material in contact with a flow battery's negative electrode. Titanium coordination complexes containing at least one catecholate ligand (i.e., titanium catecholate complexes) can be particularly useful for this purpose. From various commercialization standpoints, aqueous production methods for titanium catecholate complexes using low-cost materials can be highly desirable. Conventional routes for preparing titanium catecholate complexes typically involve the use of various reactive titanium (IV) sources, such as titanium oxysulfate, titanium tetrakis(isopropoxide), and titanium tetrachloride. The former two compounds are rather expensive and are not well suited for commercial scale operations as a result. Titanium tetrachloride, in contrast, is relatively inexpensive, but the high water reactivity of this substance has conventionally precluded its utilization as a starting material in aqueous methods for forming titanium coordination complexes. In particular, titanium tetrachloride reacts with water under typical aqueous conditions to form hydrogen chloride and titanium dioxide, the latter of which is not considered to be a suitable precursor for forming titanium coordination complexes.
In view of the foregoing, aqueous methods for forming titanium coordination complexes, particularly titanium catecholate complexes, using low-cost starting materials would be highly desirable in the art. The present disclosure satisfies the foregoing need and provides related advantages as well.